Despite their generally low discharging potential of approximately 2 V, lithium sulfur batteries have received attention as a battery for next generation electric vehicles due to their excellent safety, low active material costs, and discharging capacity of 2,600 Wh/kg.
Lithium-sulfur batteries are a secondary battery normally using sulfur series compounds having sulfur-sulfur bonds as a positive electrode active material, and using alkali metals such as lithium, or carbon-based materials capable of intercalation and deintercalation of metal ions such as lithium ions as a negative electrode active material, and store and generate electric energy using an oxidation-reduction reaction reducing an oxidation number of sulfur (S) as sulfur-sulfur (S—S) bonds are broken during a reduction reaction (discharge) and forming sulfur-sulfur (S—S) bonds again as an oxidation number of sulfur (S) increases during an oxidation reaction (charge).
However, such lithium sulfur batteries have a problem in that lifespan characteristics are reduced due to a phenomenon of losing lithium polysulfide formed in a positive electrode during charge and discharge reactions outside the positive electrode reaction area. A Li—S battery forms a polysulfide intermediate when charged and discharged. The polysulfide is eluted in an electrolyte diffusing to a negative electrode surface, and reacts with the negative electrode to form insoluble Li2S and Li2S2. Sulfur used as a positive electrode active material is lost due to such a reaction and battery performance declines, and such a phenomenon is referred to as a shuttle effect.
Specifically, in a lithium sulfur battery, sulfur-sulfur chemical bonds during discharge are gradually cut and transferred to sulfur-lithium bonds, and lithium polysulfide (Li2Sx, x=8,6,4,2) formed in the middle readily bonds with a hydrophilic solvent as a material having strong polarity. The lithium polysulfide dissolved in an electrolyte may diffuse in the form of LiSx or anions (LiSx−, Sx2−), and when the lithium polysulfide diffuses from a sulfur positive electrode, the lithium polysulfide escapes from an electrochemical reaction area of the positive electrode decreasing an amount of sulfur participating in the electrochemical reaction in the positive electrode, which resultantly causes a capacity loss. There are also problems in that lithium polysulfide reacts with a lithium metal negative electrode through continuous charge and discharge reactions, and lithium sulfide (Li2S) is fixed on the lithium metal surface, and as a result, reaction activity decreases and potential characteristics become inferior.
Technologies having been used in the art to solve such a problem of lithium polysulfide loss in lithium sulfur batteries are largely divided into 3 technologies. The first method is delaying a positive electrode active material outflow by adding an additive having a property of adsorbing sulfur to a positive electrode mixture, and herein, examples of the used additive may comprise active carbon fiber, transition metal chalcogenides, alumina, silica and the like. The second method is surface treating a sulfur surface with a material comprising hydroxides, oxyhydroxides of coating elements, oxycarbonates of coating elements or hydroxycarbonates of coating elements. The third method is preparing a carbon material as a nanostructure to restrain lithium polysulfide in a nano-structured capillary tube.
However, in the above-mentioned technologies, the method of adding an additive adsorbing sulfur to a positive electrode has a problem of electroconductive deterioration and has a risk of battery side reaction caused by the additive, and the method is not preferred in terms of costs as well.
The technology of surface treating a sulfur surface with a certain material has a problem of losing sulfur during the treatment process, and also has a disadvantage of requiring high costs.
Lastly, the method of preparing a conductor as a nanostructure has problems in that the manufacturing process is complicated and high costs are required, a battery volume capacity loss occurs due to the volume occupied by the carbon nanostructure, and in addition thereto, the nanostructure may lose its function during a rolling process of a battery manufacturing process.